earth science

Mélange

Mélange is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mélange rather than just read about it. In short: In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of varied lithologies.

Key takeaways

Mélange belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mélange to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mélange from memory before moving on to harder problems.

Reference excerpt

In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically consists of a jumble of large blocks of varied lithologies. Both tectonic and sedimentary processes can form mélange. Mélange occurrences are associated with thrust faulted terranes in orogenic belts. A mélange is formed in the accretionary wedge above a subduction zone. The ultramafic ophiolite sequences which have been obducted onto continental crust are typically underlain by a mélange. Smaller-scale localized mélanges may also occur in shear or fault zones, where coherent rock has been disrupted and mixed by shearing forces.

Large-scale melanges formed in active continental margin settings generally consist of altered oceanic crustal material and blocks of continental slope sediments in a sheared mudstone matrix. The mixing mechanisms in such settings may include tectonic shearing forces, ductile flow of a water-charged or deformable matrix (such as serpentinite), sedimentary action (such as slumping, gravity-flow, and olistostromal action), or some combination of these. Some larger blocks of rock may be as much as 1 kilometre (0.62 mi) across. Before the advent of plate tectonics in the early 1970s, it was difficult to explain mélanges in terms of known geological mechanisms. A particularly troubling paradox was the occurrence of blueschist blocks (low temperature and high pressure metamorphic rocks) in direct contact with graywacke (a coarse sandstone with lithic fragments) that was deposited in a sedimentary environment.

Examples

Examples include the Franciscan Complex along the Coast Ranges of central and northern California and the Bay of Islands ophiolite complex in Newfoundland. The Gwna Mélange in the UK extends through Anglesey and the Llŷn Peninsula onto Bardsey Island in North Wales. The Northern Palawan melange is distributed in the Philippines' Miniloc Island, west coast of Inabamalaki Island, west coast of El Nido; Cudugman Point on Bacuit Bay, and in the Cuyo Group of Islands. It consists of a jumble of various rock types contained in a matrix of grey-green slaty mudstone and siltstone. The Eastern Desert of Egypt is part of the Neoproterozoic Arabian-Nubian Shield and displays different occurrences of Neoproterozoic ophiolitic mélanges. The mélanges contain exotic and native blocks and fragments of variable sizes and types set in a sheared and schistose volcaniclastic matrix. The main exotic blocks are ophiolitic and include metamorphosed ultramafic rocks, metagabbros, massive and pillowed metabasalts and pelagic sedimentary rocks. Based on the mode of occurrences of the ophiolitic components and the processes of mélange formation, the ophiolitic mélanges of the Central Eastern Desert are classified and mapped into tectonic mélange, olistostrome and olistostromal mélange. Both tectonic and sedimentary processes played a major role during mélange formation in a back-arc or inter-arc setting.

Etymology The term mélange in English is a loan word from French, used to mean a mixture of disparate components. Its derivation, and therefore to some extent its connotation, is similar to mêlée. Mélange is the modern form of the Old French noun meslance, which comes from the infinitive mesler, meaning "to mix".

References

Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 178, 514), ISBN 0-7167-2438-3. Hsu, K.J., 1970, Preliminary report and geologic guide to Franciscan melanges of the Morro Bay – San Simeon area, San Luis Obispo County, California: California Geological Survey Special Publication 35. Raymond, L.A., 1984, Classification of melanges: Geological Society of America, Special Paper 198, pp. 7–20. British Geological Survey: "Geology of the country around Aberdaron," HMSO, London (1993), ISBN 0-11-884487-3

External links

Trenches and mélanges from the University of Alberta

Illustrations

Worked examples

Example 1 — a first encounter with Mélange

Start with the simplest possible case. Write down what Mélange claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mélange before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mélange ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mélange

In research: Mélange appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mélange in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mélange is common in secondary-school and first-year university syllabi. It links to neighbouring topics Petrology, Structural geology, so understanding it makes those chapters shorter.
In everyday life: Look for Mélange outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mélange in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mélange means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mélange out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mélange in simple terms?

Why does Mélange matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mélange?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mélange.